This is a double-blind trial to evaluate the hypothesis that the beneficial effect of low-dose Methotrexate therapy in rheumatoid arthritis is due to the inhibition of aminoimidazole-carboxamide-ribotide transformylase, a folate-dependent enzyme which catalyzes the last step in the de novo biosynthesis of inosine monophosphate. Patients are supplemented in double-blindfashion with either folic and or folinic acid in a double-blind fashion and urinary levels of aminoimidazole carboxamide and adenosine are monitored.